Agatha De Lacey

Agatha first appears anonymously (described only as "a young creature") in
the Creature's narration (2.3.6):

I ate my breakfast with pleasure, and was about to remove a plank to
procure myself a little water, when I heard a step, and, looking through a
small chink, I beheld a young creature, with a pail on her head, passing
before my hovel. The girl was young and of gentle demeanour, unlike what I
have since found cottagers and farm-house servants to be. Yet she was
meanly dressed, a coarse blue petticoat and a linen jacket being her only
garb; her fair hair was plaited, but not adorned; she looked patient, yet
sad.

She provides the Creature with his first experience of beauty (2.3.7):

The young girl was occupied in arranging the cottage; but presently she
took something out of a drawer, which employed her hands, and she sat down
beside the old man, who, taking up an instrument, began to play, and to
produce sounds, sweeter than the voice of the thrush or the nightingale.
It was a lovely sight, even to me, poor wretch! who had never beheld aught
beautiful before.

The Creature first learns her name in 2.4.3, as he learns the rudiments of
language. Over time he comes to know her history: she once "had ranked
with ladies of the highest distinction" (2.6.1), but after De Lacey's fall, she was
imprisoned with her father (2.6.5).
After five months in prison, the family was condemned to exile, and found
"a miserable asylum in the cottage in Germany," where the Creature first
encounters her.

In her last appearance in the novel, she faints from terror upon
beholding the Creature (2.7.10):

At that instant the cottage door was opened, and Felix, Safie, and Agatha
entered. Who can describe their horror and consternation on beholding me?
Agatha fainted; and Safie, unable to attend to her friend, rushed out of
the cottage.

When the Creature returns to the cottage, the De Laceys have deserted it.